The major mineralized tissues, bone and dentin, are comprised of a continuous organic matrix in which the discontinuous mineral phase becomes embedded. The organic matrix appears to direct, or at least regulate, the pattern of deposition of mineral crystals, control their size and modulate their stability to dissolution. The mechanism by which the matrix exercises these functions is not clear but appears to be related to specific interactions between, and localization of, particular matrix components. Our objective in this program is to understand the nature of this set of interactions and relate this to the biological control of bone and dentin mineralization. Our previous work suggests that one of the key factors is the interaction of the matrix collagen with an acidic protein at the mineralization front. In dentin this acidic protein is a phosphoprotein (phosphophoryn). There is also a phosphoryn-like protein in bone. Further work can be considered in terms of several specific problems: 1.) Characterization of the matrix phosphoprotein in bone and dentin; 2.) Study of the chemistry of the phosphoprotein-collagen interaction; 3.) Development of an organ culture system and study of the biosynthesis of the phosphophoryns and collagen in dentin and bone; and 4.) The role of non-collagenous proteins of the matrix in stimulating bone formation and bone induction.